Warming Autumns May Hinder Plants' Climate-Cooling Role

Brian Handwerk
for National Geographic News

January 2, 2008

As Earth warms and seasons shift, plants and soil may become less efficient at keeping the greenhouse gas carbon dioxide out of the atmosphere, a new study suggests.

Earth's so-called carbon sinks—soil, vegetation, and oceans—currently absorb about half of the carbon dioxide that humans produce by burning fossil fuels. (See how the greenhouse effect works.)

While some data suggest that warmer springs in the Northern Hemisphere would allow plants and earth to absorb more carbon, new research suggests that warmer autumns would offset or even trump any such gains.

"Warming autumn will enhance carbon loss from terrestrial ecosystems," said lead author Shilong Piao of the Laboratory of Climate and Environmental Sciences in Gif-sur-Yvette, France.

During photosynthesis—the process by which plants convert sunlight into energy—plants absorb carbon and combine it with water to produce carbohydrates.

But vegetation and soil also produces carbon dioxide during the decay of plant matter and during respiration, in which plants use oxygen to create energy and release CO2 as a waste product.

In warm months, when growth rates are high, an ecosystem with lots of vegetation may act as a greenhouse gas-trapping carbon sink. But during colder periods, respiration can outweigh absorption, and the same plants, and the soil they grow in, may become net sources of atmospheric carbon dioxide.

If springs and autumns continue to warm, longer growing seasons could alter the absorption rates of each season—with uncertain results for the total amount of atmospheric CO2.

John Miller, of the National Oceanic and Atmospheric Administration in Boulder, Colorado, says the effect of warmer autumns may have to do with soil moisture. "In the fall, soil is typically drier, and perhaps the plants are done with the majority of their growth," he said.

"So the warmth in fall may be benefiting the microbes in the soil, which are busy spewing out CO2 as they chew on old plant parts, much more than the plants."

Fifty Percent "Discount" Discontinued?

Piao and colleagues studied ten different Northern Hemisphere sites to arrive at their findings, which are published in this week's issue of the journal Nature.

But they caution against generalizing too much based upon the results.

"There are many quite different ecosystems in the world, and different ecosystems may show different responses of [the] carbon cycle to autumn warming," Piao noted.

"Further, more detailed studies are needed to quantify historical changes in [the] seasonal carbon cycle for each ecosystem."

Miller concurred.

"I think our current understanding of how earlier springs and/or later falls affect carbon uptake and release is quite weak," he said.

While scientists can accurately measure the "bottom line" levels of carbon entering the atmosphere, they don't fully understand all the variables that produce this result and how they change for different plant species and types of land.

"If we ever want to predict how climate change affects CO2 sources and sinks, we need to better understand how climate changes—including temperature, precipitation, soil moisture, and humidity—affect photosynthesis and respiration individually," Miller added.

But Piao's results do raise a disturbing possibility: We may no longer be able to count on plants and oceans to absorb 50 percent of our emissions as the planet continues to warm.

"It's possible that warming temperatures could help plants absorb more of our C02 emissions than they currently do, particularly if they somehow accelerate growth without also boosting decomposition," Miller allowed.

But, he stressed, models suggest that exactly the opposite will happen, and what he dubs the "50 percent discount" could be diminished.

"Most models predict that the land biosphere will start to lose carbon faster than it gains it."

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